When OsmoBSC uses libosmo-sigtran for the A interface transport, it misses the capabiliy to transport/encapsulate the MGCP messages inside the IPA multiplex (like used towards osmo-bsc_nat). This must be ported over from old OsmoBSC.

During a recent discussion it became aparrent how important this is for deployments behind NAT. There's one IPA/SCCPlite connection between BSC and BSC-NAT, and that connection must handle BSSMAP, CTRL, MGCP and possibly more.

Architecture-wise, we'll have to add some mode in which CTRL (in libosmoctrl) can register/bind to a sub-flow of the libosmonetif IPA stream creted by libosmo-sigtran. Not yet sure how it will look in the end.

Assigning this to neels as he's doing SCCPlite interop testing. After having completed native SCCPLite, it is getting time to test against the osmo-extended IPA multiplex of bsc-nat, where this issue becomes relevant.